Method for monitoring solder paste printing process

ABSTRACT

The invention relates to a method for monitoring the paste printing process in the setting and soldering of a circuit board. In the paste printing process, solder paste ( 5 ) is spread on the circuit board ( 4 ) at the surface mounted devices to be set or at the corresponding solder pads ( 7 ) of the connecting pins. According to the invention, at least one paste test pattern ( 9 ) is arranged on the circuit board ( 4 ), which test pattern consists of a number of test elements ( 91, 92, 93, 94 ), shaped like geometrical plane figures on the surface of the circuit board, and which test elements have varying degrees of difficulty in view of the printing process.

This invention relates to a method according to the preamble of theindependent claim for monitoring the solder paste printing process inthe setting and soldering of a circuit board.

Most of the electronic components used today are surface mounteddevices. The components are placed on the surface of the circuit boardat predetermined places with regard to the wiring pattern. Before theinstallation of the surface mounted devices, a paste-like solder alloyis printed on the surface of the circuit board at the solder pads of thecomponents. After this, the surface mounted devices are assembled on thecircuit board. The components stay where they are put, because theconnection pins of the components stick to the paste deposits of thesolder pads. When the components have been assembled, the circuit boardsare arranged to go through a furnace where the paste deposits melt. Whenthe circuit boards cool down, the paste hardens and the surface mounteddevices have been fastened on the surface of the circuit board by meansof solder joints.

The above described solder paste printing process is very sensitive andrequires constant monitoring and control in order to achieve the optimumsolder quality. It is also to be noted that the size of the componentshas continuosly decreased, and thus also the size of the terminal pinsand solder pads and the gaps between them have decreased, andconsequently the requirements for the solder paste printing process havealso increased.

The solder paste printing process has earlier been monitored randomly byexamining the paste printed circuit board with a microscope. With thenaked eye, without any instrument, it is difficult or nearly impossibleto perceive a difference in the coverage area of paste deposites at thesolder pads of normal components, even if the difference isconsiderable. The object of the invention is to provide a new method formonitoring the solder paste printing process in the component settingand soldering of a circuit board.

According to the invention, in the method for monitoring the solderpaste printing process in the setting and soldering of a circuit boardin order to evaluate the quality of the paste printing process, at leastone paste test pattern is applied on the circuit board. The test patternconsists of a number of test elements, shaped like geometrical planefigures arranged on the surface of the circuit board, which testelements are of varying degree of difficulty in view of the printingprocess.

The inventive idea is to use geometrical test patterns printed withpaste, such as rectangular test patterns, as the indicators of thecapability of the solder paste printing process. The shape and/or sizeand/or gaps between the test elements used in the test patterns variessystematically in a manner such that the test patterns have variousdegrees of difficulty in printing. The test figures can be classifiedaccording to the degree of difficulty in printing, and according to theclasses of difficulty, the solder paste printing process used can beclassified into predetermined classes in inspection, and the solderpaste printing process can be given a ratio that describes itscapability.

In this connection, the term circuit board is used to refer to circuitboards in general, meaning both the actual circuit board on which thecomponents are set and soldered, and a circuit board billet which cancomprise one or more actual circuit boards. The solder paste testpattern can be arranged on the actual circuit board, if it has room forit, or on the circuit board billet outside the actual circuit board(s)arranged on it. The solder paste test pattern can be arranged separatelyfrom the solder pads of the actual components, or, if it has beenarranged on the actual circuit board, its test elements can includesolder pads used for fastening the components. Thus the test patterncan, in some applications, be arranged among the solder pads so that thesolder pads are used to constitute at least a part of the test pattern.

An advantage of the method according to the invention is the fact thatby means of it the solder paste printing process can be reliablymonitored and verified. When the test pattern is used, random checks ofthe printed solder pads of the surface mounted devices before thesoldering process can be eliminated.

Another advantage of the method according to, the invention is the factthat it can be used to determine the capability of the solder pasteprinting process and the accuracy of the parameters used in the printingprocess. These factors can have a considerable improving effect on thequality of the soldering process. Consequently, inspection of thecircuit boards with surface mounted devices after the process can begiven up or at least substantially reduced.

Furthermore, it is an advantage of the invention that the same testpattern can be used in the production of many kinds of circuit boards,when surface mounted devices are used in the process.

Due to the invention, the monitoring process in the manufacture ofcircuit boards becomes less complicated. For example, one part of thetest pattern, such as one test element, can be used after the solderingas a test point which is connected to an electric circuit. On the otherhand, non-contacting inspection methods, such as a camera system, canalso be used.

In addition, due to the invention it is possible to get comparablemonitoring results of the manufacture of circuit boards, regardless ofwhich product is manufactured on the circuit board production line. Themethod according to the invention also provides a possibility forautomatic monitoring of the solder paste printing process. It is to benoted that since the production line is used to manufacture differentcircuit boards for different products, with different arrangements ofsolder pads, there has not been a single, comparable monitoring methodfor these circuit boards available earlier, but the monitoring criteriahad to be defined for each product individually.

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 shows a cross-section of the solder paste printing process;

FIG. 2 shows an enlarged cross-section of the solder pad on which thepaste deposit has been applied in the paste printing process;

FIG. 3 shows one advantageous test pattern for estimating the quality ofthe paste printing process;

FIG. 4 shows another advantageous test pattern for estimating thequality of the paste printing process;

FIG. 5 shows a circuit board panel which comprises many circuit boards,in which circuit board billet a test pattern has been applied; and

FIG. 6 shows a diagrammatic plan of automatic monitoring equipment forthe paste printing process.

In this invention report, setting and soldering of the circuit boardmeans the arrangement and fastening of components, which transmit and/orprocess electricity and/or electric signals, on the circuit board, whichitself contains the wiring between the components and possibly alsoconnections to peripheral devices.

The electronic components used today, the VLSI circuits in particular,are most often surface mounted devices (SMD).

These components are set on the surface of the circuit board. Thecircuit board contains the wiring which connects various components andother electric devices, such as connectors. In order to attach thesurface mounted devices on the surface of the circuit board, apaste-like solder alloy is printed at the solder pads which correspondto the connecting pins. Paste deposits of a suitable size are thusformed at the solder pads. After this, the surface mounted devices areset on the circuit board. The components stay where they are put on thecircuit board, because they stick to the paste deposits of the solderpads at their connecting pins. After the setting, the circuit boards arearranged to go through a hot furnace, where the paste melts and thecomponents are fastened in place at the connecting pins. When thecircuit board cools down, the paste hardens and the solder joint at thesolder pad of each connecting pin is complete.

The principle of the surface mounting process is illustrated in theFIGS. 1 and 2. A printing mask or stencil 1, which is a thin metalplate, for example, is used in the surface mounting process. Openings 2have been etched on the stencil 1 at the solder pads 7 reserved for theconnecting pins of the surface mounted devices. The stencil 1 isfastened to a frame 3. In the solder paste printing process, the stencil1 fastened to the frame 3 is placed on the surface of the circuit board4 or in close vicinity to it at a precisely specified point so that theopenings 2 match the solder pads 7. Solder paste 5 is applied on theupper surface of the stencil 1. The paste 5 is spread with a specialspatula 6 over the circuit board 4 so that the paste fills the openings2 of the stencil 1 and is squeezed onto the surface of the circuit board4 at the predetermined solder pads 7. When the stencil 1 is lifted offfrom the circuit board 4, the paste remains as deposits 8 of suitableheight h on the surface of the circuit board 4 at the openings 2 andsolder pads 7.

The solder pads 2 of the surface mounted device on the circuit board 4are generally rectangular areas, the size of which corresponds to thesize and shape of the connecting surface of the connecting pins of thecomponent. The solder pads 7 are arranged at distances which correspondto the distance of the connecting pins of the components and in a shapethat corresponds to the shape formed by the connecting pins of thecomponent.

The coverage area of the paste deposit 8 depends on the width and lengthof each opening 2 of the stencil 1. In addition, the height of the pastedeposits 8 depends on the thickness of the stencil 1 (height h of pastedeposit, see e.g. FIG. 2). The size of the openings made in the stencil1 is determined by the size of the solder pad spots in the photographicfilm used in the manufacture of the stencil. The solder pad spots areusually a little smaller than the solder pads, but when the openings 2are etched in the stencil 1, they are generally slightly overetched andthus they become the size of the solder pads 7. If it seems that theamount of paste applied on the solder pad 7 with the stencil 1 is notappropriate, the solder pad spots of the photographic film can bedecreased or increased according to need, and a new stencil 1 withopenings 2 of the right size can be made.

The purpose of the solder paste printing process is to achieve aright-sized paste deposit 8 on the solder pads 7, or a sufficient amountof paste; not too much and not too little, so that the connecting pinscan be fastened by a solder joint reliably on the solder pads 7 reservedfor them and related to the wiring. The method of the invention is usedparticularly for verifying the right size of the paste deposits.

When the FIG. 3 is examined, it can be seen that a paste test pattern 9,separate from the actual solder pads 7 of the components, has beenapplied on the circuit board 4, which test pattern 9 comprises a numberof geometrical test elements 91, 92, 93, 94. These test elements are ofvarying degree of difficulty in printing.

The test pattern 9 in FIG. 3 consists of four rectangular test elements91, 92, 93, 94, which are of different widths, denoted by a, b, c and d,respectively. In addition, the test elements 91, 92, 93, 94 are arrangedin a straight line in sequence at different distances from each other ina manner such that the distance between the test elements 91, 92 is e,the distance between 92 and 93 is f, and the distance between 93 and 94is g. The relation between the test elements 91, 92, 93 and 94 ispreferably the following: a<b<c<d. The relation between the distances ofthe test elements 91, 92, 93 and 94 is preferably the following: e>f>g.Furthermore, the test elements 91, 92, 93 and 94 are in this applicationessentially rectangular in shape.

In another advantageous embodiment of the invention, the test elementsof the test pattern 10 are arranged in two sets 11, 12, as shown in FIG.4. Both sets consist of test elements which are essentially rectangularin shape and of different widths. Set 11 consists of the elements 111,112, 113, 114, and set 12 consists of the elements 121, 122, 123 and124. Both sets 11, 12 can be like the test pattern 9 shown in thepreceding FIG. 3. The test elements 111, 112, 113, 114; 121, 122, 123,124 of both sets 11, 12 are thus arranged in sequence in a straight lineat different distances from each other. In this application, the sets11, 12, or rows of test elements are at an angle, preferably a 90° inrelation to each other.

When the test pattern 10 is formed of two sets of test elements 11, 12,which are arranged at an angle in relation to each other, as shown inFIG. 4, possible changes of the paste printing process can be looked atboth horizontally and vertically, or aligned with the direction of theprinting process (moving direction A of the spatula 6) andperpendicularly against it. Thus any possible defects of the printingprocess in both main directions can be checked with the same testpattern.

The openings of the test pattern 9, 10 are made in the stencil 1 in thesame way as openings 2 are generally made in it for the solder pads 7(cf. FIG. 2). By means of the paste printing process, paste testingareas or test patterns 9, 10, separate from the normal solder pads 7,are applied on the circuit board, which test patterns can then be usedfor assessing the quality of the paste printing process. Paste depositsaccording to the test pattern 9, 10 can be examined either visually (bymicroscope etc.) or by means of separate monitoring equipment.

FIG. 5 shows a circuit board billet 13, on which a number of actualcircuit boards 14, 15, 16, 17 have been arranged. In this case, a pastetest pattern 18 according to the invention can preferably be arranged onthe circuit board billet 13 outside the actual circuit boards 14, 15,16, 17. Thereby, when the circuit boards are complete, the results ofthe paste printing process can be simply checked from the test pattern18 arranged on the circuit board billet, which test pattern will beeventually disposed of when the circuit boards have been detached fromthe billet 13. The test pattern 18 can be similar to the test pattern 10in FIG. 4, for example.

When the paste printing process is monitored by the method according tothe invention, the paste test patterns 9; 12; 18 are monitored,particularly the coverage area (width a, b, c, d) and/or height (h) ofthe test deposits according to the test elements 91, 92, 93, 94; 101,102, 103, 104, 111, 112, 113, 114 of these test patterns and the gaps e,f, g between the test elements of these test patterns. Thus all kinds ofchanges in the paste printing process can be monitored by means of thetest patterns.

If the widths a, b, c, d of the test elements 91, 92, 93, 94 of the testpattern 9 have been chosen appropriately, as shown in FIG. 3, forexample, the results of the monitoring can be classified according tohow well the paste printing process has succeeded for each test element91, 92, 93, 94. If the smallest test element 91 has an area of just therequired size and the paste deposit in it has the right amount of paste,the printing process can be regarded as successful in all respects; thepaste printing process belongs to the first or best printing class. Aprerequisite for this is that the printing of the other test elements92, 93, 94 has also succeeded. If the first test element 91 of the testpattern 9 has failed and the other test elements 92, 93, 94 have passed,the paste printing process can be classified as belonging to the second,or second best, printing class. Similarly, on the basis of the printingresult of different test elements 92, 93, 94, the whole printing processcan be classified into subclasses that reflect the printing quality.

The gaps between the test elements of the test pattern, such as the gapse, f, g, between the test elements 91, 92, 93, 94 can also be used toevaluate the capability of the paste printing process. The pass/failstatus of the gaps can be determined on the basis of the gaps being freeof paste or partly filled by it. If all the gaps between test elements91, 92, 93, 94 are free of paste, the printing process can be classifiedinto the first or best quality class. If the gap between the testelements 93, 94, with a distance g, is at least partly filled by paste,the quality of the printing process does not meet the requirements ofthe first class but it belongs to the second, lower quality class.Similarly, the quality classification can be lowered, if paste is foundduring inspection in the gaps between the test elements 92, 93, 94, withdistances f, e.

The quality of the paste printing process can also be monitoredautomatically by means of suitable monitoring equipment, as is shown byan illustrative example in FIG. 6. The monitoring equipment 20 compriseone or more camera(s) 21, such as a semiconductor camera which isequipped with a suitable lense arrangement 22, and a monitoring unit 23,to which the camera 21 is connected. The monitoring unit 23 consists ofa memory unit 24 and a processing unit 25. The shape of test patterns tobe arranged on the circuit boards, such as the pattern 9 of FIG. 3,including e.g. the shape/s and dimensions of the elements of the testpattern, have been saved in the memory unit 24. Suitable applicationprograms for automatic testing have also been saved in the memory unit24. The test results, such as an alarm of process disturbances, aregiven out from the monitoring equipment 20 through a suitable connection26 for handling by the monitoring personnel.

The monitoring equipment 20 function in the following manner, forexample. The circuit boards 27, which have passed through the pasteprinting process, are arranged to go through the observation field andexposure area of the camera 21 on a suitable conveyer in a manner suchthat the test pattern 28 can be photographed by the camera. Theprocessing unit 25 compares the test patterns 28 of the circuit board 27photographed by the camera 21 to the shapes of test patterns saved inthe memory unit 24 by means of the application programs, and observesthe differences on the basis of which the quality of the paste printingprocess is evaluated in accordance with agreed criteria, such as thecriteria described above.

In the FIGS. 3 and 4 above, various test patterns 9; 10, 11, have beenpresented by way of example. It is to be noted, however, that the testpatterns can differ from those presented. The test pattern can consistof test elements with different basic geometrical shapes, such asrectangle, square, circle etc. These test elements can be arranged atdifferent distances from each other. The test elements of a test patterncan contain parts of mutually different width. In general, it can besaid that the paste test pattern consists of geometrical plane figures,which can vary and have gaps of different width between them, so thatdifferent parts of the test pattern have different degrees of difficultyin view of paste printing.

What is claimed is:
 1. A method for implementing a process formonitoring the solder paste printing process in the setting andsoldering of a circuit board, in which paste printing process solderpaste (5) is spread on a circuit board (4) at the solder pads (7) ofsurface mounted devices or corresponding connecting pins, characterizedin that for evaluating the quality of the paste printing process atleast one paste test pattern (9; 10; 18) is arranged on the circuitboard (4; 13), which test pattern is constituted by a number of testelements (91, 92, 93, 94; 111, 112, 113, 114; 121, 122, 123, 124), theshapes of which correspond to geometrical plane figures on the surfaceof the circuit board, which test elements have varying degrees ofdifficulty in view of the printing process, and wherein the spatialrelationship between test elements is such that the distance between onetest element and another is different and each of said test elements hasdifferent width and the distance between consecutive elements is smalleras the widths of each said test element becomes larger.
 2. A methodaccording to claim 1, characterized in that the test pattern isconstituted by test elements of different shapes, corresponding togeometrical plane figures.
 3. A method according to claim 1,characterized in that the test elements (91, 92, 93, 94) of the testpattern (9) are arranged at different distances (e, f, g) from eachother.
 4. A method according to claim 1, characterized in that the testelements (91, 92, 93, 94) contain parts of mutually different widths (a,b, c, d).
 5. A method according to claim 1, characterized in that thetest pattern (9) is constituted by test elements (91, 92, 93, 94),essentially rectangular in shape, which have different widths (a, b, c,d).
 6. A method according to claim 5, characterized in that the testpattern (9) is constituted by four test elements (91, 92, 93, 94), whichhave been arranged at different distances (e, f, g) from each other. 7.A method according to claim 5, characterized in that the test pattern(10) consists of two sets of test elements (11, 12), each of which setsconsists of a number of test elements (111, 112, 113, 114; 121, 122,123, 124), which have been arranged at different distances from eachother and each of which sets is at an angle, preferably a 90 degreeangle, in relation to each other.
 8. A method according to claim 1,characterized in that the test pattern (18) is arranged on the circuitboard (13), which comprises several circuit boards (14, 15, 16, 17). 9.A method according to claim 1, characterized in that the paste printingprocess is monitored on the basis of the test pattern automatically bymeans of monitoring equipment (20).
 10. A method according to claim 9,characterized in that the monitoring is implemented by means ofmonitoring equipment (20) constituted by a camera (21) and a monitoringunit (23) connected to it, which monitoring unit consists of a memoryunit (24), to which the shape of the test pattern (28) has been saved,and a processing unit (25), in which processing unit (25) the testpatterns (28) photographed with the camera (21) are compared to thesaved shapes of test patterns and the differences are observed, on thebasis of which differences the quality of the paste printing process isevaluated.